1. Field of the Invention
The present invention relates to apparatuses and methods for detecting information on objects (samples), including image-obtaining apparatuses for observing the properties and shapes of objects using electromagnetic waves with high frequencies, mainly from millimeter waves to terahertz waves (30 GHz to 30 THz).
2. Description of the Related Art
Nondestructive sensing technology based on electromagnetic waves ranging from millimeter waves to terahertz (THz) waves (30 GHz to 30 THz, hereinafter simply referred to as THz waves or THz light) has recently been developed. The field of applications for electromagnetic waves in this frequency band under development includes: imaging technology for providing a safe alternative to radioscopy; spectroscopy for examining the properties of a substance, such as bound states, by determining the absorption spectrum and complex dielectric constant of the substance; analysis of biomolecules; and evaluation of carrier density and mobility.
U.S. Pat. No. 5,789,750 discusses a photoconductive switching element as an example of a suitable THz generator. This photoconductive switching element includes a substrate having a photoconductive film deposited thereon and an antenna serving as an electrode. Examples of frequently used photoconductive films include a radiation-damaged silicon film deposited on a sapphire substrate and a low-temperature-grown GaAs (LT-GaAs) film deposited on a GaAs substrate.
U.S. Pat. No. 5,710,430 discusses a method for obtaining a THz image by two-dimensionally scanning an object with photoconductive switching elements disposed on the transmitter and detector sides thereof, as shown in FIG. 9. According to this publication, the dispersion/absorption properties of an object for THz waves can be used to obtain an optically unobservable internal image such as an interconnection pattern inside an IC chip.
This scanning method, however, takes much time to obtain a two-dimensional image. Japanese Patent Laid-Open No. 2004-85359 discusses a method for inspecting an object at high speed by simultaneously irradiating the object with THz waves over a one-dimensional region. According to this publication, THz pulse light (generally having a pulse width of 3 ps or less) is shaped into a one-dimensional beam using, for example, a parabolic cylindrical mirror for high-speed transmission/reflection spectroscopy of objects.
Detection by the methods discussed in U.S. Pat. No. 5,710,430 and Japanese Patent Laid-Open No. 2004-85359 involves excitation (pumping) of THz pulse waves with a femtosecond laser and gating with a probe laser. These methods require a complicated, less flexible optical system to synchronize pumping light and probe light. In addition, a high signal-to-noise (S/N) ratio is difficult to achieve because the power of THz waves generated by photoconductive switching elements is generally on the order of submicrowatts. To increase sensitivity, therefore, the speed at which signals are obtained should typically be reduced, and accordingly the speed at which images are obtained is limited. Furthermore, the resolution of pulse light is limited because the light is typically susceptible to wavelength dispersion. The above methods also have limited applications because they generally require a large, expensive apparatus with large power consumption, typified by a high-power femtosecond laser. This is a bottleneck in popularizing the use of THz imaging in industry.
In addition, THz waves used in known methods for obtaining images often have a circular or oval Gaussian intensity distribution. Some THz generators generate THz waves with a multipeaked distribution, not even a Gaussian distribution. When pixels are simultaneously obtained, such an intensity distribution tends to degrade peripheral image quality.